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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Electrical pulse on telephone lines?
Summary: Automatic Line Insulation Test (ALIT) is not a TDR
Keywords: ring
Message-ID: <3570@kitty.UUCP>
Date: 23 Dec 89 03:20:31 GMT
References: <4208@crdgw1.crd.ge.com> <4910@blake.acs.washington.edu> <1967@eric.mpr.ca>
Distribution: na
Organization: Recognition Research Corp., Clarence, NY
Lines: 44

In article <1967@eric.mpr.ca>, dleblanc@joplin.mpr.ca (David LeBlanc) writes:
> >This was discussed in comp.dcom.telecom some time ago.  As I recall,
> >the explanation is that the telco sends a measured voltage pulse down
> >the phone lines periodically.  By measuring the current flow and
> >certain other things, they can check the quality of the phone line
> >and detect some physical problems as they develop (e.g. insulation
> >failure, etc.).  The phone-switch in your central office does this
> >automatically, at a time when the line is idle (usually late at night),
> >and enters a trouble-report in the database if anything unusual
> >is detected.
> 
> Yup, its technically called "Time Domain Reflectometry".

	Both of the above posters have described the operation of a TDR
(Time Domain Reflectometer).

	HOWEVER, the CO apparatus responsible for line insulation testing
(ALIT), does NOT operate on the principle of a TDR.  ALIT apparatus
measures the leakage of Tip and Ring to ground, generally using a
measuring circuit which applies DC excitation.  An optional ALIT function
is the measurement of foreign potential (FEMF) - which is indicative of
leakage resistance to adjacent cable pairs.

	The "chirp" in the el cheapo telephone ringers is caused by the
momemntary interruption of line relay (or ESS supervision) voltage,
followed by application of ALIT test voltage, following by reapplication
of line relay voltage.

	While a TDR will detect various types of cable faults, it is
generally impracticable for for routine ALIT testing.  Furthermore, the
automatic analysis of a TDR signature is complex, and there are no
off-the-shelf systems for automated telephone pair testing which employ
a TDR, anyhow.

	In addition, while a TDR is useful, it has distinct limitations
when applied to testing of cable pairs, with such limitations including
but not limited to: distance, inability to pass through more than one
loading coil, susceptibility to crosstalk interference from adjacent
working, and inability to function in the presence of FEMF leakage.

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